Evaluation of Stationary Creep Rate in Heat-Affected Zone of Martensitic 9–12% Cr Steels

The purpose of the present study was to evaluate the contribution of distinct regions of the simulated heat-affected zone (HAZ) to the overall creep behavior of welded joints in the X20 and P91 steels. The HAZ was simulated by means of dilatometry at four peak temperatures (900, 1000, 1200, and 1350 °C) with a consequent tempering at 650 °C. Microstructure features of the four simulated HAZ regions including precipitates, prior austenite grains, and subgrains were quantified by means of electron microscopy. The quantified parameters and the measured hardness were used in three physical models for evaluation of the stationary creep rate (ε˙ at 170 MPa and 580 °C. The resulting ε˙ values fall within the range 10−8–10−7 s−1, being in good agreement with the experimental data with a similar thermal history, but an order of magnitude lower than the measured values for the parent metal of the studied steels (10−7–10−6 s−1). Depending on the model utilized, their output can be linearly related to hardness, subgrain size, or interparticle spacing. The model relating ε˙ to hardness was the most consistent one in prediction, being always lower for higher peak temperatures.

The Influence of Laser Nitriding on Creep Behavior of Ti-4Al-4V Alloy with Widmanstätten Microstructure

Ti-6Al-4V alloy has been considered in applications of aeronautical and aerospace industries, due to its properties such as high specific resistance, good creep resistance and metallurgical stability. However, its use in applications for high temperatures is restricted due to its great affinity with the oxygen, which results in the formation of oxide layers and limits its mechanical resistance at these conditions. Thus, specific treatments have been employed in the material to work as surface barriers to avoid the oxygen diffusion in the alloy under high temperature conditions. One surface treatment that can be used is laser nitriding. In the present work, the surface of Ti-6Al-4V alloy with Widmanstätten microstructure was nitrided by applying Nd:YAG laser focal with 0.6 mm diameter, at laser power of 700, 750 and 800 W, process speed of 100 mm/s and 20 L/min of N2 flow. Creep tests were performed at constant load at 600 °C and 125 MPa, to verify the influence of treatment on the Ti-6Al-4V alloy. Results have indicated a lower stationary creep rate for the titanium alloy with Widmanstätten laser-nitrided structure when compared to the non-nitriding material. Besides that, the surface hardness increased from 368 HV of base material to 1000 HV after laser nitriding.

Ageing effect on the creep performance of simulated weld HAZ for the steels X20 and P91

Changes in size- and spatial distribution of precipitates and their effect on stationary creep rate of two grades of creep-resistant 9-12% Cr steels, X20 and P91, as a function of ageing at two different conditions were investigated. Ageing of both steels was performed at 650°C lasting up to 2 years and at 750°C lasting up to 6 months. Prior to the ageing, simulation of two weld heat-affected zone HAZ regions, i.e., coarsegrained microstructure (γ), and grain-refined microstructure (α+γ) was performed. Accelerated creep tests at 580°C and 170 MPa, lasting up to 100 hours were performed on the above microstructures prior to- and after the ageing. In addition, optical and SEM imaging were performed on metallographic specimens, followed by image analysis. Deterioration of the creep performance show a proportional behaviour with the precipitate coarsening, which is more pronounced after the ageing at 750°C. Comparatively, the increase of the stationary creep rate after the ageing at 650°C was more pronounced in the steel X20 than in the P91. In both steels and both tempering temperatures, the most troublesome region with respect to the creep behaviour was observed to be the simulated intercritical (α+γ) HAZ microstructure.

Study on Creep Characteristics of Cemented Waste Rock Backfills

Slip instability and tensor instability are two kinds of failure modes of cemented waste rock backfills. In order to simulate the creep characteristics of cemented waste rock backfills, PFC was carried out. The results show that: rocks inside the models will have displacement under load. The expansion is caused by large lateral displacement of the rocks. A non-stationary creep model of cemented waste rock backfills was established based on the results of simulation. This model can be used to describe the whole creep characteristics of cemented waste rock backfills better.

The Viscoelastic-Plastic Displacement Back Analysis of Rock Mass with Non-Stationary Parameters

The FEM positive analysis is made using H-K non-stationary creep constitutive model in this paper, the finite element program for non-stationary viscoelastic-plastic displacement back analysis is compiled combining the non-stationary viscoelastic-plastic program with the complex shape optimization method, and the displacement back analysis of soft tunnel engineering is conducted. The result indicates that the viscous aging characteristics of rock mass can be reflected objectively if rock mass is regarded as non-stationary viscoelastic-plastic model, and the plastic zone development of surrounding rock can be predicted considering the plastic flow of rock mass and regarding the back analysis results as the calculation parameters.